lowering freezer temp ?

[knighty]

quick summary... I need to freeze a lot of dog food and do it overnight (4000lb a night normally)

this summer heat is causing problems so I was thinking about cooling the high pressure line with water before it went back into the freezer.... I've done this now with a bit of a bodge... I wrapped copper tube around the hot line and tape'd it on bloody tight... then pumped cold water through it.... but rough, but it works well, can feel the temp difference between before and after just by hand, and temps pull down much faster now

so that will work for now...

but soon all the dog food to be frozen will need to be stored in this freezer... so the original cooler unit (from the lorry) is being repaired/converted to run from the mains all the time (it broke a while back and a normal freezer unit was added to the room)

that should help bring temps down a bit quicker, and help on the hotter days...

but I'd like to drop the min. temp down a bit, at the moment it won't go below -20'C... is there anyway I can cheaply/easily drop that temp ?

I'm guessing it must be set by the expansion valve ?

thinking at the moment is that if the room has 2 separate cooler system's on it, one could run as normal to take it down to -20... and then the other could be set lower, say -30 ?...

any input/ideas anyone ?

I'll get more information on the compressors etc... asap

[Brian_UK]

The temperature is probably more dependent on the system design and capacity or even the standard of insulation.

The expansion valve has nothing to do with the temperature acheivable.

More to follow...

[knighty]

oh... right...what set's the min tempreture then ? I was kind of under the assumption that the temp. of the evap. grill was set by the expansion valve...

the freezer in question is the back off lorry, it's 26feet long and the insulation is pretty thick, all door seals are good etc...

it will happily reach and hold -18/-19 even after adding 2500lb of fresh meat to it (to freeze) but it won't go any lower...

at -19 the suction line back to the compressor ices up, and the hot liquid comming out of the compressor isn't very hot... just a bit warm.

I was thinking that the expansion valve on the side of the evap inside the freezer sets the rate of flow through the evap and so the pressure on the low side... ? (that is the expansion valve right?)

I'm trying to work with what I have and not spend too much money... the more I spend the less I get paid !

thanks for the input brian

(I'm a geek... I know what it's like trying to help people who don;t really know what there talking about)

[knighty]

hmm... if the hot high pressure line coming out of the compressor doesn't get that hot, does that mean that maybe that at -19'C (or thereabouts) the system is undercharged ? that there's not enough pressure on the high side to dump enough heat ?

(p.s. i'm dyslexic and most of what I know about anything I thought myself... and I like tho think aloud)

[John Wood]

What refrigerant is this system on? It sounds as if the valve isn't matched to the refrigerant and at low temperature the superheat is fading away to zero. Be careful, the liquid going back to the compressor might damage it.

[SteveDixey]

it will happily reach and hold -18/-19 even after adding 2500lb of fresh meat to it (to freeze) but it won't go any lower...

Is the meat already fully frozen (i.e. no soft stuff in the core of the product) or is above freezing point?

If it freezes the stuff then that takes a big chunk of power so the plant must be reasonably fit.

If you are putting it in already frozen, then you might have individually or in partnership:

Not enough refrigerant flow to remove the heat (i.e. undercharged) or pipes not big enough to flow the refrigerant required to remove the heat
Heat coming in through the cabinet walls and doors
Some system component not flowing enough refrigerant to cool the product any further
Product packed too close together so air cannot circulate
In sufficient temperature difference between the refrigerant and the product. If you want -20 then may need -28 to -30 air off the evaporator

Steve

[knighty]

hi guys, thanks for the replies !

some meat is stored in the freezer already frozen, but the 4000lb I mentioned is the amount of fresh stuff we need to freeze each night !

at the moment theres not normally more than 1000lb of fresh meat in this fridge.

the fridge works pretty well, when it used to be set to -18'C it would bring the temp down pretty quick and hold it there, once it was down to temp the compressor would hardly run.

when I check for min. temps I always check on a Monday morning when the freezer has has 36 or so hours of un-interrupted running with no-one opening the door etc...

I'm not sure which gas it's running with, I'll get back to you on that

going by what you guys are saying, and that fact that the compressor hardly runs once the room is down to -18'C, I take it that it "should" be able to go colder than that ?

[Andy]

hi guys, thanks for the replies !

some meat is stored in the freezer already frozen, but the 4000lb I mentioned is the amount of fresh stuff we need to freeze each night !

at the moment theres not normally more than 1000lb of fresh meat in this fridge.

the fridge works pretty well, when it used to be set to -18'C it would bring the temp down pretty quick and hold it there, once it was down to temp the compressor would hardly run.

when I check for min. temps I always check on a Monday morning when the freezer has has 36 or so hours of un-interrupted running with no-one opening the door etc...

I'm not sure which gas it's running with, I'll get back to you on that

going by what you guys are saying, and that fact that the compressor hardly runs once the room is down to -18'C, I take it that it "should" be able to go colder than that ?

Hi Knightly

by running the freezer at a lowertemperature,it will not freeze anymore product, infact lessasyou willbe requiring a lower product temperature whichs takes more running.

Meat freezes in three stages

sensible cooling above freezing

latent cooling

sensible cooling below freezing

The latent phase which starts and finishes at the same temperature, about -4 deg c (takes about 100 times the energy per lb of each of the sensible cooling regions)

If you had a freezer with only fresh product and a 24 hr freezing cycle, after the first hour you would enter the latent phase,it would stay there for the next 20 hours at -4 deg c before dropping.

Hope this helps.

Kind Regards Andy

[knighty]

ahh, I didn't think about that... I know the same principle when applied to water... to freeze it you have to drop it down to 0'C then it will stay there for a while even tho you keep pulling heat out of it untill it finally converts from a liquid to a solid ?

if the 20hours a rough guess kind of figgure... I can normally freez most of the meat in less time that that, it's just when I put too much in one freexer it struggles.... ok there's more total heat to pull out of the room, but part of it is that it's oftern packed in a lot tighter (not much I can do about that at the moment)

if I can use just one freexer for all of it, I can have a much nicer setup... with fans blowing over the trays of meat....

I tried to take some photos to get modle numbers etc... but I must have messed up because most of them are out of focus

[knighty]

the photos.... thats the freezer I'd like to use for freezing everything down from fresh to frozen... (with another freezer unti added to it)

it's a bit full in there at the moment... I'm planning to empty the back of the freezer and have all the racks there, with fan's blowing over them... access to the racks via the back roller door - which backs right to some doors on the factory where there meat will be coming out of when my new packaging machine gets delivered !

everything in the boxes is already frozen... there's pallets on the floor and up against the back door to help circulation.... I'd like to add some spaceing round the sides too, but I think that might have to wait a while untill I have some more time.

there's 30lb of meat on each tray in the racks... tho sometimes when were busy there'll be 40lb
at the moment I'm doing my best to arange production so out busiest days are at the end of the week when the meat can be left to freez over the weekend, of to have a quiet day after a busy one to do the same.

I'll pick up one of those infa-red thermomitors so I can get some system temp's, and I'll get someone I know with guages to come past and get some pressures too !

thanks.

Alan.

p.s. sorry about the spelling guys, I'm dyslexic and I'm not on my machine at the moment.... and this one doesen't have a spell checker !

[Andy]

Hi Knightly

Probably not enough head height in the room, but when building rack/trolley freezers, we would fit a false ceiling.
This is basically a set of fridge panels running horizontal in frount of the cooler (like an extension of the drip tray). The cold air off the cooler is blown over the false ceiling (the racks are wheeled under the false ceiling) the air then returns through the racks bck to the cooler (small 1 foot sections of plastic curtain are hung under the false ceiling to stop the air taking a short cut back to the cooler above the racks).

This set up will make a big difference to your freezing time (provided that no short cuts are left for the air, ie gaps around and above the racks, or racks left out of the rows).

Hope this helps

Kind Regards Andy

[Andy]

Hi Knightly

Probably not enough head height in the room, but when building rack/trolley freezers, we would fit a false ceiling.
This is basically a set of fridge panels running horizontal in frount of the cooler (like an extension of the drip tray). The cold air off the cooler is blown over the false ceiling (the racks are wheeled under the false ceiling) the air then returns through the racks bck to the cooler (small 1 foot sections of plastic curtain are hung under the false ceiling to stop the air taking a short cut back to the cooler above the racks).

This set up will make a big difference to your freezing time (provided that no short cuts are left for the air, ie gaps around and above the racks, or racks left out of the rows).

Hope this helps

Kind Regards Andy

Please see attached, I hope it comes out alrights.

Air flow is from left to right, from the cooler to above the false ceiling (the cooler shown here is a special with the fans on the return air, most commercial coolers have the fans on the air off).


Kind Regards Andy

[knighty]

hi andy

thanks for the input

do you think building something simmiler to that at the back end of the fridge (where I'll be moveing all the racks too) would still be a big help ? ... if it's not the air right out of the evap. but just the air from the room blowing around ?

that sounds like a much better system that my idea of fans right in the way to blow the air around...

I was thinking of hanging a curtain wall thingie in the freezer anyway to section off between the "storage" area and the "freezing" area... the door to the freeing area will be open a lot during the way and I don't want to lose all the cold from the entire room !

[Andy]

hi andy

thanks for the input

do you think building something simmiler to that at the back end of the fridge (where I'll be moveing all the racks too) would still be a big help ? ... if it's not the air right out of the evap. but just the air from the room blowing around ?

that sounds like a much better system that my idea of fans right in the way to blow the air around...

I was thinking of hanging a curtain wall thingie in the freezer anyway to section off between the "storage" area and the "freezing" area... the door to the freeing area will be open a lot during the way and I don't want to lose all the cold from the entire room !

Hi Knightly

it would need to be the air off the evaporator, as it will take pressursed air to be able to force it's way through the racks.

The curtain would be a good idea, but you will need a separate cooler in that area to cool the product.

If you have a lot of door openings build an anti-room at the door from fidge panels and hang a chill cooler, running at -3 deg c return air. This will remove the most of the moisture load from the air and stop the coolers freezing up.

The dual use of a freezer/blast room has one big problem, the moisture ingress from the door openings

If you can eliminate this you will gain a lot of extra capacity.

So to re-cap.

1/ blast freezer inside the freezer with false ceiling.

2/ curtain off the holing freezer area and hang one of your spare units

3/ add an anti-room and chill to -3 degc

This will give you the best results for your type of application without spending mega bucks.

Remember when not blasting at the weekend you can allow the holding freezer to keep your product in the blast without having to transfer it

Possibly the best way to fit the false ceiling in you room is to raise the roof and the cooler and add in a couple of foot to the wall panels.

Hope this helps

Kind Regards Andy

[knighty]

thanks andy, your input is a big help

I'll have a look at the room tomorrow and see what I can do, I'm a little tight on freezer room at the moment to implement all of those, but hopefully in the future everything going to plan I'll be able to all those things

for now... I'll do as much as I can

[Andy]

thanks andy, your input is a big help

I'll have a look at the room tomorrow and see what I can do, I'm a little tight on freezer room at the moment to implement all of those, but hopefully in the future everything going to plan I'll be able to all those things

for now... I'll do as much as I can

Rome wasn't built in a day

Good Luck.

Kind Regards Andy

[Samarjit Sen]

Hi Knighty,

You are getting a temperature down to - 18 Deg. C. and now you require a temperature of - 30 deg. C. I hope I got it right. If that be the case, then you have to redesign your complete system and calculate the heat load at the required temperature and select the equipments that is also the compressor accordingly. Please note that for storage of meat you also have to maintain the RH. Trying to experiment with the existing system you will not only wasting money but it will not work. I have been in the low temperature application field for a long time and have executed a number of similar projects.

I shall be happy to help you, ofcourse should you require, but then you will have to provide me with all the relevant datas.